How to use SUMO Block Jointing Mortar (BJM) for AAC blocks
Thick sand-cement mortar wastes AAC blocks’ best feature — their precision. Thin-bed block jointing mortar lays them faster, straighter and stronger. Here is how to use SUMO BJM.
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AAC (autoclaved aerated concrete) blocks are made to tight tolerances, so they do not need thick, uneven sand-cement beds. A thin-bed block jointing mortar (SUMO BJM) uses a 2–3 mm joint, which is faster to lay, uses far less material, and gives a stronger, straighter wall.
This guide covers mixing BJM to the right consistency and laying AAC blocks thin-bed so the wall goes up quickly and true.
What you’ll need
Step by step
Lay a true first course
The first course sets the whole wall. Bed it dead level and to line on a conventional levelling mortar — everything above depends on this course being right.
Mix BJM to a smooth paste
Add SUMO BJM powder to clean water and mix with a paddle to a lump-free, creamy but firm paste. Let it stand briefly and remix. Mix only what you can lay before it starts to stiffen.
Prepare the blocks
Brush dust off the block faces. Lightly dampen very dry blocks so they do not rob water from the thin mortar bed, but do not soak them.
Comb on a thin bed
Apply BJM to the bed and vertical joint face with the notched applicator to leave an even 2–3 mm ribbed layer. Thin and even is the goal — this is not a thick levelling bed.
Lay, tap and check
Place each block, tap down with a rubber mallet to the string line, and check level and plumb as you go. The thin bed grabs quickly, so line the block up as you set it.
Keep courses true
Check level, plumb and line every few courses. Clean off squeeze-out before it sets, and let the wall gain strength before loading it.
Care, tips & what to avoid
- !Using BJM as a thick levelling bedIt is a 2–3 mm thin-bed adhesive. A thick bed wastes material and weakens the joint. Level the first course conventionally instead.
- !A wonky first courseThin-bed walls cannot hide a bad start. Lay the first course dead level and to line.
- !Over-wetting or soaking blocksSoaked blocks and a sloppy mix both weaken the joint. Lightly dampen very dry blocks only.
- !Mixing too much at onceBJM stiffens as it stands. Mix batches you can lay in the working time.
